Even as negotiators scramble to salvage an agreement at the foundering Copenhagen climate talks, a new study in this week's issue of Nature shows that the consequences of inaction on reducing emissions could be more severe than anyone thought.

By looking back about 125,000 years, to a time when global temperatures were as high as they are expected to be by 2100, a team of scientists from Princeton and Harvard universities has calculated that the oceans were probably at least 26 ft. higher than they are now, maybe as much as 31 ft. higher. That's significantly higher than the 13-ft.-to-19-ft. range scientists have been counting on, and it is, write Peter Huybers of Harvard and Peter Clark of Oregon State University in an accompanying commentary in Nature, "a disconcerting message." (See the top 10 green ideas of 2009.)

One reason for the higher estimate is simply that scientists had more data to work with. Most studies of ancient sea level focus on a specific area of the globe. But local sea levels, then and now, do not give a true picture of the global average sea level, which is what really matters. Lots of factors can affect regional sea-level variability, including winds and local currents that push water consistently toward or away from a particular shore. "One of the biggest effects," says the study's lead author, Robert Kopp, who did his research during a postdoctoral fellowship at Princeton, "is gravity." The world's giant ice sheets, such as Greenland's, are so massive that they actually pull the oceans toward them, raising sea level in the surrounding region. "If you were in Scotland, and Greenland started melting," he says, "local sea level would actually fall at first."

Also, climate scientists had assumed that the Antarctic ice sheets would have remained intact during that long-ago warm period. Because of changes in Earth's orbit, they know there would have been more sunlight hitting the Arctic back then, which means less sunlight in the Antarctic, and so, presumably, less melting.

Evidently not, though. To get a true picture of worldwide sea levels, Kopp and his colleagues gathered data from a wide range of individual studies to put together a global picture. "We reviewed data from 40 sites," he says, including evidence from ancient coral reefs, eroded beaches and telltale sediments laid down in the ebb and flow of 125,000-year-old tides. The bottom line: local effects and faulty assumptions may have led to an underestimate. "It's unclear," says Kopp, "why the Southern Hemisphere would have been warmer than we thought." It may have to do with changes in ocean circulation, but nobody knows at this point.

A 26-ft. rise in sea level would be truly catastrophic if it happened by the end of this century. But there is no suggestion in the study that the rise is imminent. "We can only give a thousand-year average," says Kopp, meaning that it might well take a millennium for sea level to go up that much. The rise would be inevitable, though: even if we cut back emissions today, concentrations of greenhouse gases will continue to increase, albeit more slowly. As a result, if temperatures go up by as much as 2°C (3.5°F) by the end of the century  the upper limit of temperature rise that climate scientists consider safe  they're likely to stay that high for a long, long time, further increasing the risk of rising seas.

And there's no guarantee that the rise in sea level would necessarily be smooth. If ice sheets begin sliding into the sea faster than they have in the recent past  as they seem to be doing already  sea level could go up more quickly than average, reaching a catastrophic point relatively early, then staying there. "From our analysis," says Kopp, "we really can't know how long it would take." In short, the science is still uncertain  but less so than before, and moving in a direction that isn't reassuring.